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1985 Front Disc Rotor Size


davidjonh
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I have finally been able to drive my electric Avanti with a total mileage of about 30 miles so far. However, I just discovered that I had the front left brake hanging up. During my rebuilding of things I had replaced the rotors, calipers, pads and brake lines. After taking the brake components apart I discovered that the bracket to which the caliper is mounted was bent and slightly twisted. I can't be sure if I just didn't notice this when I was redoing everything or if this happened while I have been recently test driving the car. What does appear a little strange is that the pads on both sides sit about 1/4 inch above the top of the rotor. What I am now wondering is are the rotors the wrong size. They measure 10 1/2 inches. Thinking back a long time ago I seem to remember that the wear pattern on the old pads showed the pads wearing such that those pads must have also been just a little higher than the rotor. I would appreciate any comments.

Dave

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Rotors are supposed to be 11.5"....if they're the original Dunlop/Bendix design brakes on the front of your car. If you have to replace the rotors it's much more cost effective to install the Turner conversion kit.

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Thanks for the response. I probably would have considered the Turner kit but when I was repairing these things probably three years ago I was unaware of most things about Avantis. I'm not sure what was originally on the car when I got it except I do know that the wheel rims are Ford with a Ford bolt pattern (actually a Lincoln Towncar rim). I'm thinking the bracket holding the caliper is original. While 11.5 inch rotors have been mentioned to me recently after this problem developed I'm also pretty sure the extra inch in diameter would not fit into the existing bracket--an 11 inch rotor might just fit but I have no idea if one is made for my application. I purchased the rotors from a place in Sacramento which deals almost completely in brakes and clutch components and thought they would know the size of the rotor for my application. However, I can't remember if I took the original rotors in, which might have been the wrong rotor, and asked to replace them.

Is there a way to determine if these are the original Dunlop/Bendix designed brakes? Being an 85 I thought they were GM parts. I looked on the net for the number which was on the receipt for the caliper and it did seem to match a 1985 Avanti.

Thanks again, Dave

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Avanti Motors didn't go to the GM platform and brakes until the '87 model year. You have to have the original Stude pattern since you have Ford rims. If it was a GM platform it would be a 4 3/4" bolt circle rather than 4 1/2".

Original calipers of the Dunlop/Bendix design would have "Bendix" on the caliper housing. It could also have "Sumitomo" cast into the caliper...they're the same application as some Datsuns and maybe Toyota sports cars of the mid 1960's used them. My '70 had Sumitomo marked calipers on it before I switched to the Turner kit. Apparently Avanti Motors sourced them from whatever vendor had them at the best price at any particular time.

Again...if you find you have to replace the rotors and pads...new original pattern rotors cost $200 each...maybe $25-$30 for pads. The Turner kit...rotors, pads, calipers, bearings, seals, etc., costs $650. For the difference in cost it's worth it. You may be able to sell the old parts (if they're good) and offset some of the conversion costs.

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Thanks again. The original rotors and calipers, or what was on the Avanti when I got it, were in such bad shape I tossed them a number of years ago. The new calipers don't have any names on them and I am assuming they are just a replacement part. I found my original receipt and the number on it corresponded to what you can find on the net for an 85 Avanti.

Your are absolutely correct, and I had never thought about it, the fronts are at 4 1/2 for a Ford. I remember that when I was redoing the rear brakes on the 84 Corvette rear end that the bolt pattern was different so I either needed to change the hub bolt pattern or the rim.

Been talking to another great Avanti guy and looking more at the Turner kit and it seems that the turner kit has a double thickness in the center area where the bracket would be the most likely to bend or twist. Since a picture may help explain what I now have I'm going to try to include a few. From looking at the Turner brackets I'm wondering if I could use these brackets with the components I now have with my 15 inch rims. Otherwise, if things bend again I might try to re-enforce the center section of the bracket.

Brake-bracket-without-rotor.jpg

Brake-bracket-front.jpg

Brake-bracket-back.jpg

post-2770-1297107817896_thumb.jpg

post-2770-12971078470056_thumb.jpg

post-2770-12971078723016_thumb.jpg

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  • 4 weeks later...

I just wanted to provide an update in case it might help others with similar issues. I want to thank both Dan Booth and Jim Turner for their help provided in identifying the parts and for future solutions. As it turns out I sent pictures to Dan and he indicated that the bracket and rotor I have are basically the original 1985 design. Dan indicated it was normal for the pads to sit above the rotor edge and that after approximately 30,000 miles this resulted in some Avantis pulling to the left or the right as the overhang of the worn pad would rub on the top of the rotor. To solve this he would taper the top edge of the pad so as the pad wears there is no overhang produced. He also indicated that if I could bend the bracket back to within about 0.010 to 0.020 inch I might be able to keep the brake from locking up. I did my best with what I had in my garage and bent the bracket back and tapered the top of the pads. So far the brake is not locking up and it seems my problem has been solved for now.

Considering the extra weight in the front of my electric Avanti and the lack of compression to slow down an electric vehicle, a long term solution in my case is to install the Turner brake system with larger rotors and pads that contact more of the rotor surface area. As Jim and a lot of others have pointed out to me, the original design with the pad edge sitting above the rotor edge seems a little strange but again apparently it is normal.

Thanks again to everyone for your help, especially Roland Vardon for pointing me in the right direction. I now have about 50 electric miles on the Avanti but now have another problem with the drive train as I snapped a one inch adapter shaft which was keeping the Corvette rear end from lifting under acceleration.

Dave Heacock

Edited by davidjonh
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